FIG. 5 shows a circuit in which a load is connected to the collector of a transistor. The collector of a load driving transistor 511 receives a voltage from a power source 515 via a load 513. The base of the transistor 511 receives an input voltage via a resistor 517.
If the load 513 is short-circuited for some reason, a great current will flow between the collector and emitter of the transistor 511 and destruct the transistor 511.
A countermeasure against such a destruction is to interpose a fuse in series with the load 513. However, the current interrupting time of the fuse upon an abnormal current is more than several seconds, and the circuit is not recovered by simply removing the cause of the short circuit, unless the blown fuse is replaced by a new fuse.
In a prior art circuit shown in FIG. 6, a resistor 611 is interposed between the emitter of the transistor 511 and the earth (common potential line) to prevent destruction of the driving transistor 511. When the load 513 is short-circuited, the voltage drop in the resistor 611 increases, but the current flowing in the transistor 511 is maintained substantially constant by a feedback to the base (fixed current circuit). However, the circuit invites a great increase of consumption in the transistor 511, and therefore requires a large-scaled heat sink or an extremely large-capacity transistor with respect to the capacitance of the load.